Re: [PATCH 1/2] ftrace: disable preemption on the testing of recursion

From: 王贇 <yun.wang@linux.alibaba.com>
To: Steven Rostedt <rostedt@goodmis.org>, Miroslav Benes <mbenes@suse.cz>
Cc: Guo Ren <guoren@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	Helge Deller <deller@gmx.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Jiri Kosina <jikos@kernel.org>, Petr Mladek <pmladek@suse.com>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Colin Ian King <colin.king@canonical.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Jisheng Zhang <jszhang@kernel.org>,
	linux-csky@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-riscv@lists.infradead.org, live-patching@vger.kernel.org
Subject: Re: [PATCH 1/2] ftrace: disable preemption on the testing of recursion
Date: Wed, 13 Oct 2021 09:52:16 +0800	[thread overview]
Message-ID: <6ca59cdd-e378-b8c6-7ac6-472568f1af97@linux.alibaba.com> (raw)
In-Reply-To: <20211012082920.1f8d6557@gandalf.local.home>


On 2021/10/12 下午8:29, Steven Rostedt wrote:
> On Tue, 12 Oct 2021 14:24:43 +0200 (CEST)
> Miroslav Benes <mbenes@suse.cz> wrote:
> 
>>> +++ b/kernel/livepatch/patch.c
>>> @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>>>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>>>  	if (WARN_ON_ONCE(bit < 0))
>>>  		return;
>>> -	/*
>>> -	 * A variant of synchronize_rcu() is used to allow patching functions
>>> -	 * where RCU is not watching, see klp_synchronize_transition().
>>> -	 */
>>> -	preempt_disable_notrace();
>>>
>>>  	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
>>>  				      stack_node);
>>> @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>>>  	klp_arch_set_pc(fregs, (unsigned long)func->new_func);
>>>
>>>  unlock:
>>> -	preempt_enable_notrace();
>>>  	ftrace_test_recursion_unlock(bit);
>>>  }  
>>
>> I don't like this change much. We have preempt_disable there not because 
>> of ftrace_test_recursion, but because of RCU. ftrace_test_recursion was 
>> added later. Yes, it would work with the change, but it would also hide 
>> things which should not be hidden in my opinion.
> 
> Agreed, but I believe the change is fine, but requires a nice comment to
> explain what you said above.
> 
> Thus, before the "ftrace_test_recursion_trylock()" we need:
> 
> 	/*
> 	 * The ftrace_test_recursion_trylock() will disable preemption,
> 	 * which is required for the variant of synchronize_rcu() that is
> 	 * used to allow patching functions where RCU is not watching.
> 	 * See klp_synchronize_transition() for more details.
> 	 */

Will be in v2 too :-)

Regards,
Michael Wang

> 
> -- Steve
>